1. Field of the Invention
This invention relates to the field of well logging and, more particularly, to well logging techniques and apparatus for determining formation properties, such as resistivity, at several different radial depths of investigation and with compensation for factors such as borehole rugosity and drift of electronic components. The invention has general application in the well logging art, but is particularly useful in measuring while drilling.
2. Description of the Related Art
A commonly used technique for evaluating formations surrounding an earth borehole is resistivity logging. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while porous formations with low resistivity are generally water saturated. However, the region immediately surrounding the borehole can be invaded by borehole fluid or mud filtrate and have a different resistivity than the virgin formation. If a resistivity logging device has only one radial depth of investigation, there is limited ability to measure resistivity of all zones of interest, and there may be difficulty in determining if the measured resistivity represents the invaded zone, the virgin zone, or some combination of the two zones. However, if the resistivity logging device has multiple radial depths of investigation, there is greater flexibility. In addition to the advantage of having, for example, a shallow measurement and a deep measurement individually, the combination of the two can provide additional information such as the extent of invasion. It is also possible to combine two or more measurements, for example a shallow measurement and a deeper measurement, to compute a better estimate of the true formation resistivity.
Wireline resistivity logging tools have long been provided with two or more radial depths of investigation. Conventional wireline resistivity logging tools typically achieve two depths of investigation by using a short and a long vertical array of electrodes or coils. In general, a long vertical array provides a greater radial depth of investigation than does a short vertical array. More recently, as will be discussed momentarily, measuring while drilling tools have been provided with multiple radial depths of investigation.
A type of well logging which is of interest herein is so-called electromagnetic propagation logging, which can be used to measure the resistivity of the formation surrounding a borehole. For example, U.S. Pat. No. 3,551,797 describes a technique wherein electromagnetic energy is transmitted into the formation, and energy which returns to the borehole is measured at a receiver pair to determine the attenuation and/or the phase shift of the electromagnetic energy propagating in the formation. More than one vertical spacing between a transmitter and different receiver pairs may be used to obtain different radial depths of investigation. For example, a receiver pair relatively close to the transmitter can be used to obtain attenuation and/or phase shift information from which the properties of the invaded zone are determined, and measurements of the attenuation and/or phase shift from a receiver pair relatively far from the transmitter can be used to obtain the properties of the deeper uninvaded formations. Either attenuation or phase shift can be used to determine a bulk value of the electromagnetic skin depth for the formation, with the bulk conductivity then being determinable from the electromagnetic skin depth.
Various other techniques also exist in the art for utilizing multiple transmitters and/or receivers to investigate resistivity at different depths of investigation.
In U.S. Pat. No. 4,899,112 there is disclosed a logging apparatus for determining resistivity of formations at two different radial depths of investigation using signals received at only a single receiver pair. The resistivity of formations at a relatively shallow depth of investigation around the receiver pair is determined as a function of the phase shift measured at the receiver pair, and the resistivity of formations at a relatively deep depth of investigation around the receiver pair is determined as a function of the attenuation measured at the receiver pair. The apparatus is particularly advantageous for measurement while-drilling, where it is desirable to obtain resistivity at multiple depths of investigation while minimizing the length and complexity of the logging device. U.S. Pat. No. 4,899,112 also discloses a so-called borehole compensated embodiment, where the receiver pair is located between and equally spaced from, a pair of transmitting antennas that can be alternately energized. The signals received at the receiver pair can be averaged to obtain borehole compensated signals; i.e., signals from which (1) drift of electronic components, and (2) borehole rugosity, have been reduced or removed by cancellation.
One or more additional transmitters can be added to the logging device just described, at different spacing(s), to attain further depths of investigation. However, in order to provide borehole compensation for each additional spacing, it would be necessary to provide two transmitters (one on each side of the receivers) for each such additional spacing.
It is among the objects of the present invention to provide a logging device and technique that determines formation properties at several depths of investigation, with the advantage of borehole compensation and also with minimal use of antennas.